Type this question into a chatbot and you get two kinds of wrong answer. The wellness answer says yes, your watch tracks your happy chemicals — it doesn’t. The skeptic’s answer says no, that’s just heart rate — which misses what the signal actually carries. The honest answer sits in the middle, and it is more interesting than either.
Why can’t a wearable measure dopamine directly?
Because levels live in the body’s chemistry, not on its surface. Neurotransmitter and hormone levels are measured in blood draws, spinal fluid, or brain imaging. No optical sensor on a wrist reads a molecule. Any product that says it measures your dopamine is selling the lab result without the lab.
What does the cardiac signal actually carry?
The effects. Dopamine and oxytocin act on the heart through the nervous system — dopamine tracking attention, oxytocin tracking emotional resonance — and their activity leaves a signature in the heart’s rhythm. The molecule stays invisible; what it does to the heart’s rhythm does not.
That distinction — levels versus effects — is the whole game. You cannot count the chemical from the wrist. You can read what it is doing to you, second by second, while the moment is still happening.
How was this validated?
The measurement comes out of roughly twenty-five years of neuroscience on how the brain’s valuation of an experience shows up in the cardiac signal. The strongest recent test: a 2024 peer-reviewed study read the cardiac signal from an ordinary wrist wearable, once per second, as people went about normal days — and from that signal identified the days people themselves rated as low-mood with 90% accuracy. Not from a lab, not from a questionnaire in the moment. From the rhythm.
What can you do with a chemical’s effects?
More than with its level. A level tells you about your state; the effects, read continuously, tell you about your moments — which hour held your attention and moved you, and which one just passed through you. That is a measure of what an experience was worth to your brain.
That is the measurement brainful — the b-score app for Whoop straps — is built on: it reads the effects of those chemicals on your body from the signal your strap already sends, and puts a b-score (0–100) on each calendar event, on the phone. The missing metric for Whoop: not what the day cost you, but what each hour was actually worth.
Sources: Merritt, S. H. & Zak, P. J. (2024), “Continuous remote monitoring of neurophysiologic Immersion accurately predicts mood,” Frontiers in Digital Health; Immersion’s published materials on the neuroscience of attention and emotional resonance. brainful is not a medical product, and nothing here is medical advice.